1. Field of the Invention
The field of the invention relates generally to the equalization of communication signals, and more particularly to the equalization of digital quadrature amplitude modulated signals.
2. Background
Quadrature Amplitude Modulation (QAM) systems transmit information from a transmitter to a receiver using quadrature amplitude modulated (QAM) signals. QAM signals include "symbols" where each symbol typically represents a plurality of data bits. To form the symbols, two carrier signals that have a quadrature phase relationship to each other are amplitude modulated by the transmitter. After amplitude modulation, the transmitter typically combines the two modulated carrier signals. The resulting QAM signal is then modulated with a communications carrier and transmitted over a transmission channel to provide to the receiver a received signal that includes received symbols. The receiver processes the received signal to obtain the information that it contains. Quadrature amplitude modulated signal as used herein shall refer to signals that have been quadrature amplitude modulated even when the signal has been subsequently processed. Thus, quadrature amplitude modulated signal shall refer to the signal prior to modulation with the communications carrier, the signal transmitted by the transmitter that has been modulated with the communications carrier, the signal received by the receiver prior to any processing, and to the signal that has been digitized, demodulated, equalized and/or gone through detection, for example.
The transmission channel over which the QAM signal is transmitted ideally provides to the receiver a received QAM signal and received QAM symbols that are identical to the transmitted signal and the transmitted symbols. Unfortunately, transmission channels are rarely, if ever, ideal. A variety of noise disturbances and other interference associated with the transmission channel can affect the signal. For example, the transmission channel typically will have a non-ideal frequency response and will have a limited bandwidth, possibly introducing both amplitude and delay distortion. Intersymbol Interference (ISI) often occurs. Transmission at a symbol frequency Fb (also called symbol rate) that is more than twice the bandwidth of the channel typically causes intersymbol interference. ISI can also occur when there are a number of propagation paths between the transmitter and the receiver each having different time delays associated with them. When the difference between the longest time delay of a propagation path and the shortest time delay of another propagation path becomes comparable to a symbol period, intersymbol interference results.
For years communications engineers have been seeking new techniques and analyzing old techniques to reduce or eliminate the effects of intersymbol interference introduced by transmission channels. Filters called equalizers have been used in an effort to achieve this result. Unfortunately, conventional equalization techniques used to combat ISI can be relatively expensive and can add complexity to the system, particularly in high performance or high rate systems. High rate systems, for example, may require expensive equalizers that are capable of handling a large number of multiplications per second.
Accordingly, there has been a need for a method and apparatus that appropriately equalizes a noisy or otherwise distorted QAM signal without the expense and complexity sometimes associated with conventional equalization systems.